#include <stdio.h>
#include <math.h>

double diedai(double x2) {
	double x1,  f, k;
	do {
		x1 = x2;
		f =   x1 * x1 * x1 - 5 * x1 * x1 + 16 * x1 - 80;
		k = 3 * x1 * x1 - 10 * x1 + 16;
		x2 = x1 - f / k;
	} while (fabs (x1 - x2) >= 0.001);
	return x2;
}

int main() {
	double m;
	scanf("%lf", &m);
	printf("%.3f", diedai(m));
}
